With the development of communication devices such as mobile phones, increased attentions have been drawn to circuit devices configured to handle microwaves whose frequency ranging from several hundred megahertz to several gigahertz, and further millimeter waves whose frequency ranging from several tens of gigahertz to several hundred gigahertz. To handle signals in a band from the frequency range of the microwave to the frequency range of the millimeter wave, it is very important to reduce signal loss. For this reason, the circuit devices configured to handle the signals in the band from the frequency range of the microwave to the frequency range of the millimeter wave need low-loss transmission lines.
For example, monolithic microwave integrated circuits (MMICs) require transmission lines such as microstrip lines formed on a substrate on which semiconductor elements are provided. Such a microstrip line is configured such that a ground conductor serving as a ground plane faces a signal line with a dielectric film provided therebetween. For example, a semi-insulating substrate such as gallium arsenide (GaAs) is used as a dielectric, a signal line is formed on a principal surface of the substrate, and a back surface of the substrate is metallized to form a ground conductor, thereby forming a microstrip line. Alternatively, when a substrate is, for example, a silicon semiconductor, either benzocyclobutene (hereinafter referred to as BCB, relative dielectric constant 2.65) or polyimide (relative dielectric constant 3.3) which has a low relative dielectric constant is deposited on the substrate to have a thickness of several micrometers to several tens of micrometers, and the obtained film is used as a dielectric film, thereby forming a microstrip line (for example, see Patent Document 1).
Moreover, power amplifiers for a band of several hundred megahertz to several gigahertz used for, for example, base stations of mobile phones require internal matching circuits for transforming impedance in packages. Such an internal matching circuit generally includes a distributed constant circuit using a transmission line which uses, for example, a ceramic substrate having a signal line formed on its principal surface and a ground conductor formed on its back surface.